Capacity
在.NET中List的容量应该只是受到硬件限制。
属性Capacity的真正含义，是创建List时给它预分配的容量。
一旦项的数量超过了当前的Capacity，Capacity会以
Capacity = (Capacity==0)?A_VALUE_YOU_CANNOT_SURE:(Capacity*2)
的方式增长。
当使用默认构造函数创建一个List时，Capacity就是0。（或者你非要用一个0作为capacity参数调用构造）
所以当Capacity是0时，我们为List添加了第一个项目后，Capacity是多少呢？
对，是A_VALUE_YOU_CANNOT_SURE。
不过，在.NET2.0和.NET4.5中，这个值看上去都是4。
System.Diagnostics.Debug.WriteLine(string.Format("default capacity is : {0}", new List<string>().Capacity));
int initCapacity = ;
List<string> l = new List<string>(initCapacity);
System.Diagnostics.Debug.WriteLine(string.Format("constructe a list with capacity: {0}", l.Capacity));
for (int u = ; u < ; ++u)
{
System.Diagnostics.Debug.WriteLine(string.Format("Count: {0}, Capacity: {1}", l.Count, l.Capacity));
//l.TrimExcess();
//System.Diagnostics.Debug.WriteLine(string.Format(" TrimExcess() -> Count: {0}, Capacity: {1}", l.Count, l.Capacity));
l.Add("");
}
输出
default capacity is :
constructe a list with capacity:
Count: , Capacity:
Count: , Capacity:
Count: , Capacity:
Count: , Capacity:
Count: , Capacity:
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TrimExcess()
TrimExcess()的意思就是，如果Capacity大于Count，清掉哪些没有使用的空间。基本上可以理解成让Capacity等于Count，只是基本上，不能信赖这一点。.NET毕竟还要考虑很多性能问题。TrimExcess method does nothing if the list is at more than 90 percent of capacity——这就是.NET的方式。实际上把上面那段代码中的注释放开，就会看到确实存在Capacity和Count不等的情况（然而，并不是90%哟）。Capacity不够以后的增长规则，仍然是*2，并不会因为被Trim过就只是加1。
放开注释后的输出
default capacity is :
constructe a list with capacity:
Count: , Capacity:
TrimExcess() -> Count: , Capacity:
Count: , Capacity:
TrimExcess() -> Count: , Capacity:
Count: , Capacity:
TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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发现并不是90%以后
System.Diagnostics.Debug.WriteLine(string.Format("default capacity is : {0}", new List<string>().Capacity));
int initCapacity = ;
List<string> l = new List<string>(initCapacity);
System.Diagnostics.Debug.WriteLine(string.Format("constructe a list with capacity: {0}", l.Capacity));
int initCount = ;
for (int u = ; u < initCount; ++u)
{
l.Add(u.ToString());
}
for (int u = ; u < initCount; ++u)
{
System.Diagnostics.Debug.WriteLine(string.Format("Count: {0}, Capacity: {1}", l.Count, l.Capacity));
l.TrimExcess();
System.Diagnostics.Debug.WriteLine(string.Format(" TrimExcess() -> Count: {0}, Capacity: {1}", l.Count, l.Capacity));
l.Remove(u.ToString());
}
得到
default capacity is :
constructe a list with capacity:
Count: , Capacity:
TrimExcess() -> Count: , Capacity:
Count: , Capacity:
TrimExcess() -> Count: , Capacity:
Count: , Capacity:
TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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TrimExcess() -> Count: , Capacity:
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貌似条件是酱紫的
if( count<(int)Math.Floor((double)capacity*0.9) )
do trim
我也不知道我要干嘛